<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <script>
        // setInterval(function(){alert(1)},1000);
        // (function (arg) {alert(arg)})(123)
        xo = 'one';
        function f1() {
            var xo = 'two';
            function f2() {
                var xo = 'three'
                console.log(xo);
            }
            f2();
        }
        f1();
    </script>
</body>
</html>